A subsea tree is a device that is used primarily to control the flow of production fluid from a subsea well. In addition, a subsea tree may be used to direct fluid into the subsea well, such as in chemical injection.
Typically, a subsea tree will utilize several valves for controlling the flow of fluids through the subsea tree. Operation of the subsea tree valves may be controlled by a subsea control module (SCM). The SCM may include several solenoid-operated control valves that direct the flow of hydraulic fluid to the subsea tree valves. The control valves in the SCM control various operations of the subsea tree valves. The control valves are supplied with hydraulic fluid and may be controlled by electrical signals from, for example, an umbilical, which may extend from a production tree or a remote platform.
Subsea tree valves may be hydraulically-operated valves. For example, the operator for a hydraulically-operated valve may have a spring that drives the valve toward a closed state. To open the valve, a control valve must be operated to direct hydraulic fluid pressure from a source of pressurized hydraulic fluid to the valve operator to overcome the force of the spring and drive the valve towards the open state. When it is desired to return the subsea valve to its original state, the control valve is positioned so that the source of pressurized hydraulic fluid no longer directs pressurized hydraulic fluid to the valve operator. The hydraulic fluid in the operator is vented to enable the spring to return the valve to its original state.
To facilitate distribution of the hydraulic fluid in the umbilical to the SCM's control valves, the umbilical may be connected to a receptacle on a junction plate located on the subsea tree. The junction plate typically includes a hydraulic distribution line arrangement extending from the receptacle to the SCM's control valves. Where an umbilical also contains an electrical line, the electrical line can be routed from the receptacle to an electrical connection on the SCM.
At times during the life of a well, equipment must be replaced or installed or a well workover or intervention may be required. During these operations, it is key that the operation of the subsea tree be temporarily turned over to a surface workover vessel and that the production mode of operation be locked out to prevent accidental operation by sources other than the vessel when critical equipment or workover operations are underway.
To assure that the vessel has complete control of the subsea tree, an installation/workover control system (IWOCS) is typically utilized. The IWOCS includes its own umbilical that may contain both hydraulic and electrical feeds to control the subsea tree during the installation or workover operations. Typically then, the production umbilical is disconnected from the receptacle on the junction plate and parked on a seabed parking plate. This assures that the production umbilical will not accidentally operate any of the subsea tree components.
Referring to the prior art as illustrated in FIG. 1, with the production umbilical out of the way, the IWOCS umbilical 10 extending from the vessel may then be connected to the receptacle 12 on the junction plate 16. Once connected, the IWOCS umbilical 10 provides hydraulic fluid to the SCM 18 via distribution lines 20. During operation of the subsea valves, the hydraulic fluid is vented to the sea via exhaust discharge 22. Thus, hydraulic fluid must be replenished to the SCM 18 via the umbilical 10. An electrical line (not shown) can further be routed from the junction plate 16 to an electrical connection 24 on the SCM 18 as shown or a separate electrical umbilical may be used.
Another arrangement is where the control fluid power is provided by a dedicated hydraulic power pack on the ROV. In this case, the power pack must contain sufficient fluid to replenish the supply to the tree functions, as there is typically not a dedicated supply line from the surface. The requirement that hydraulic fluid in the distribution lines 20 be replenished via an internal ROV reservoir is impractical due to impact on unit size and weight and will add operational cost for retrieval time to replenish the ROV reservoir. Additionally, the discharge of fluid to the sea is obviously wasteful and may have a detrimental impact on the environment.
A need exists for a technique to solve one or more of the problems described above.